A lot of the items Marimmar mentioned will still apply. Pick up the book here and there and just flip to a section and read. Think less of memorizing all of it and more with familiarizing yourself with how skill checks in general are handled, etc. Get to know where things are in the books, when to turn to the PHB, when to turn to the DMG or even when to turn to the MM. Sometimes knowing where to look will do you more good than trying to memorize everything.
Wing it sometimes to keep from slowing the game AND then go back and look up the rules as written. For example, in the jumping into the lake with armor on you mentioned. If you don't know exactly how to handle it the first time it comes up, make a best guess. Think, "hey, armor and swimming can't be easy!" and set the DC check to what you think is appropriate at the time. Then after the game look it up on your time to see exactly how you should have handled it.
As long as you let your players know you may wing it in certain cases you should be fine. Just makes notes and at the beginning of the next session let the players know how you will be handling jumping in the lake with armor on from that point forward.
And finally, if you have the time and inclination, DM a PBeM. PBeM's by their nature are not for everyone and require a committment to keep going. But I also find as a PBeM DM it is a great way to reinforce the rules. I *always* have the time to look up a rule and do it right. This just helps make the rules more concrete in my head.
Hope some of these suggestions help. I am sure you will get many other good suggestions from these boards.